Steel powder containing phosphorus

ABSTRACT

In the manufacture of articles by compacting and sintering steel powder it is desired that the articles shall neither expand nor shrink during the sintering process. It has been found useful to incorporate into the steel powder an iron-phosphorus alloy powder having the approximate composition Fe3P, in a quantity to give the article a phosphorus content of 0.2 - 3 per cent by weight.

United States Patent [191 Lindskog et a1.

[ Sept. 17, 1974 STEEL POWDER CONTAINING PHOSPHORUS [75] Inventors: Per Folke Lindskog;Lars-1Erik Svensson, both of Hoganas, Sweden [73] Assignee: Hoganas AB, Hoganas, Sweden [22] Filed: Apr. 11, 1973 [21] Appl. No.: 349,918

[30] Foreign Application Priority Data May 2, 1972 Sweden 005754/72 [52] U.S. Cl 75/.5 BA, 75/123 D [51] Int. Cl B22f 1/00, C22c 39/54 [58] Field of Search 75/.5 BA, .5 AA, 123 D [56] References Cited UNITED STATES PATENTS 2,226,520 12/1940 Lenel ..75/214X 12/1967 Harnisch 75/.5 BA 2/1970 Dautzenberg et a1. 75/.5 BA

Primary Examiner-L. Dewayne Rutledge Assistant Examiner-Arthur J. Steiner Attorney, Agent, or FirmToren, McGeady and Stanger [5 7] ABSTRACT 9 Claims, No Drawings STEEL POWDER CONTAINING PHOSPHORUS The present invention is related to steel powder compositions containing phosphorus, which in addition to iron and phosphorus may contain one or several other alloying constituents, either alloyed or mixed with iron powder, to a total concentration of percent.

Steel powders containing phosphorus have been known at least since 1940 through e.g. the US. Pat. No. 2,226,5 20, but they have not come to extensive use in spite of the excellent mechanical properties of sintered components made from these powders. The reason for this is to be sought in phenomena which are related to the manufacturing process pressing and sintering of powder.

The demand that are put on powders for the purpose in question is above all that it should have high compressibility and that it should cause a minimum of tool wear during compaction. High compressibility means that the powder when exposed to a relatively moderate pressure achieves such a high density that the subsequent sintering results in components with satisfactory strength. This must be done without great dimensional changes during sintering (growth or shrinkage) because otherwise one may loose the greatest advantage with ferrous powder metallurgy, viz. the posssibility to produce precision components to final tolerances without machining operations.

These requirements make it difficult to use powder made by atomization of molten steel with the desired final phosphorus content, because such powders have too low compressibility. A consequence of the low compressibility is that in order to obtain components with the desired high strength it is necessary to carry out the sintering operation at such a high temperature that the shrinkage becomes untolerably great.

The above mentioned American patent circumvents the problem with compressibility by suggesting the use of finely comminuted ferrophosphorus powder with a high percentage of phosphorus mixed with steel powder of good compressibility. The use of such ferrophosphorus steel powder mixtures is, however, connected with another serious drawback, viz. excessive tool wear during compaction.

The inventors have set themselves the problem of finding a phosphorus-rich composition, which would be sufficiently brittle so that it could be comminuted to a fine powder and at the same time give rise to a minimum of tool wear during compaction. They found that the solution to this problem is to use an ironphosphorus alloy with a composition corresponding to the compound Fe P, i.e., containing 15.6 percent by weight of phosphorus.

According to the invention, steel powder containing phosphorus to be used for the production of precision machinecomponents without excessive tool wear during compaction is characterized by consisting of an essentially phosphorus-free steel powder with good compressibility and with a maximum particle size of 100 500um, and intimately mixed therewith an ironphosphorus alloy powder with a maximum particle size of 75pm at the most, preferably 45pm at the most, and with a phosphorus centent of between 13 and 17 percent, appropriately between 14 and 16 percent, i.e., preferably about the compsition of Fe P which contains 15.6 percent P and in addition thereto, if necessary, a suitable pressing lubricant such as zinc stearate in the usual proportion, the relative proportions of ironphosphorus alloy powder and steel powder being adjusted in such a way that the phosphorus content of the mixture is between 0.2 and 3 percent, preferably between 0.3 and 1.5 percent. The percentages above are in weight percent.

It is, of course, of great importance that the constituents of the powder described above do not segregate after the mixing operation. Segregation is possible during transportation of the powder from the mixer to the place where it is used and during feeding to the powder compacting press. One way of diminishing the risk of segregation is to add 50 200 g thin mineral oil per metric ton of powder continuously during filling of the mixer. This brings about an attachment of the fine particles (in this case the iron-phosphorus alloy particles) to the coarser steel particles.

A further improvement in this respect is obtained if the powder mixture is exposed to a heat treatment at 650 to 900 C for 15 minutes to 2 hours in a reducing atmosphere and subsequently the lightly sintered cake formed during the heat treatment is broken up by gentle comminution. The small iron-phosphorus alloy particles which before this treatment were loosely attached to the steel particles are now solidly bonded to the latter, which effectively prevents segregation without affecting the compressibility to any appreciable degree.

A few examples of steel powder containing phosphorus and of methods for its production as well as tensile strength and other properties obtained with sintered components made from this powder are given below.

EXAMPLE 1 Three powder mixtures A, B, and C, were prepared. The sponge iron powder with a maximum particle size of 147p.m was used for all these mixtures. The mixtures consisted of the following components:

Mixture A:

97.4 percent sponge iron powder 1.8 percent iron-phosphorus alloy powder with a phosphorus content of 25 percent and a miximum particle size of 44pm 0.8 percent zinc stearate powder. Mixture B:

96.2 percent sponge iron powder 3.0 percent iron-phosphorus alloy powder with a phosphorus content of 15 percent and a maximum particle size of 44am 0.8 percent zinc stearate powder Mixture C:

99.2 percent sponge iron powder 0.8 percent zinc stearate powder The mixture A and B thus contained 0.45 percent P, whereas mixture C was free of phosphorus.

Cylindrical compacts with a height of 8 mm, a diameter of 8 mm and a density of 6.75 g/cm were pressed. The pressing was done in an automatic press at a rate of 35 pieces per minute. The die had been made from high speed steel which had been hardened to a Rockwell C hardness of 63. The die wear was determined by measuring the diameter of the compacts at regular intervals. It was found that the diameter increased by 0.6;zm per 10,000 pieces when mixture A was used, whereas mixture B which had been made according to the invention gave an increase of 0.3;.tm per 10,000 pieces. The pure sponge iron powder C gave an increase of 0.25 pm for the same number of pieces. Mixture B thus wore the die insignificantly more than the pure sponge iron powder (C), whereas the die wear was doubled with mixture A.

The three powder mixtures were compacted into tensile test bars in a special die for this purpose using a compacting pressure of 400 MN /m The density in the green (unsintered) state was about 6.5 g/cm in all cases which shows that iron-phosphorus alloy addition did not lower the compressibility of the mixtures. The tensile test bars were sintered at a temperature of 1,l20 C for 1 hour in an atmosphere of dissociated ammonia. When they had cooled to room temperature they were tested for density, dimensional change during sintering, tensile strength and elongation. The results are given in the table below.

Three powder mixtures, D, E and F with compositions according to the table below were prepared. Mixture D:

96.9 percent atomized steel powder consisting of.0.5

percent Mo, 2 percent Ni, rest Fe, and such accessory elements that normally occur in steel, with a maximum particle size of l77um,

2.1 percent iron-phosphorus alloy powder with a phosphorus content of 14 percent and with a maximum particle size of 30am 1.0 percent zinc stearate powder 0.010 percent thin mineral oil Mixture E:

96.3 percent atomized steel powder consisting of0.5 percent Mo, 2 percent Ni, rest Fe and such accessory elements that normally occur in steel, with a maximum particle size of 177,u.m

2.1 percent iron-phosphorus alloy powder with a phosphorus content of 14 percent and a maximum particle size of 30am 0.6 percent graphite powder with an average particle size of 4p.m

1.0 percent zinc stearate powder 0.010 percent thin mineral oil Mixture F:

96.3 percent pure atomized iron powder mum particle size of l77um 2.1 percent iron-phosphorus alloy powder with a phosphorus content of 14 percent and a maximum particle size of 30am 0.6 percent graphite powder with an average particle size of 4pm 1.0 percent zinc stearate Tensile test bars were made by pressing the mixtures in a suitable die at a compacting pressure of 600 MN/m". The test bars were subsequently sintered at 1,150 C for 30 minutes in an atmosphere of partially combusted propane with a carbon potential at the sintering temperature of0.5 percent. The following results were obtained with a maxi- Mixture Density, g/cm 6.92 6.80 6.95 Dimensional change, -0.32 +0.05 +0.10 Tensile strength, MN/m 400 480 430 Elongation, 0 7 4 4 Carbon content after sintering,

The results show on the one hand that it is possible to improve the strength of the material by replacing iron powder with prealloyed steel powder in mixtures according to the invention and on the other hand that an addition of graphite also increases the strength of the sintered material. Graphite also eliminates the sometimes troublesome shrinkage effect of phosphorus.

2 kg each of mixtures E and F were filled into a funnel and the powder was then allowed to run out freely. The last tenth of each mixture to run out was collected and analyzed for phosphorus. The sample from mixture E contained 0.35 percent P and from mixture F 0.57 percent P. As mixture E, but not mixture F, contained oil, it can be concluded that the risk of segregation can be diminished by oil addition. The average phosphorus content of each of the mixtures D to F was 0.30 percent.

EXAMPLE 3 sured and the following data were obtained:

Mixture G Density, g/cm 6.52

Dimensional change, 0. l0

Tensile strength, MN/m 350 Elongation, 5

A comparison of the data for the mixtures A and B in Example 1 with those for mixture G shows that the strength of the material can be increased by an addition of copper. Furthermore, the shrinkage effect of phosphorus is diminished.

EXAMPLE 4 A mixture H was prepared in the following manner. 20 kg of an iron-phosphorus alloy powder with a phosphorus content of 15 percent and with a maximum particle size of 45pm was first mixed with kg of a sponge iron powder with a maximum particle size of l49p.m and 15 g ofa thin mineral oil. This mixture was heated to 850 C for 30 minutes in an atmosphere of dissociated ammonia. A sintered cake was formed as a result of the heat treatment, and this was comminuted to a powder with a maximum particle size of l77p.m. This powder consisted mainly of iron powder particles to the surfaces of which iron-phosphorus alloy particles were bonded. This powder was mixed with 900 kg of sponge iron powder and 8 kg of zinc stearate powder.

2 kg of the mixture so obtained were filled into a funnel and allowed to run out freely. The last tenth of the material was collected and analyzed for phosphorus. A value of 0.31 percent P was obtained. The average phosphorus content of the mixture was 0.30 percent. This result shows that the risk of segregation had been almost completely eliminated.

What is claimed is:

1. Phosphorus containing steel powder for the production of sintered precision components with a low tool wear during the compacting step, characterized in that it essentially consists of powdery steel substantially free of phosphorus and intimately mixed therewith an iron-phosphorus alloy powder with such a particle size that all of the powder passes a screen with an opening of at the most 75pm, said iron-phosphorus alloy powder having a phosphorus content of between about 12 and 16 percent by weight, the relative proportions of said iron-phosphorus alloy powder and said powdery steel being adjusted in such a way that the phosphorus content of the mixture is between about 0.2 and 3 percent by weight.

2. Phosphorus containing steel powder according to claim 1, characterized in that it also contains at the most 1.5 percent by weight of a solid lubricant powder.

3. Phosphorus containing steel powder according to claim 1 characterized in that it also contains 0.005 to 0.02 percent by weight of a thin mineral oil for the prevention of segregation.

4. Phosphorus containing steel powder according to claim 1, characterized in that the iron-phosphorus alloy particles are bonded to particles of said powdery steel by sintering and subsequent comminution for the prevention of segregation.

5. Phosphorus containing steel powder as claimed in claim 1, wherein the particle size of said ironphosphorus alloy powder is such that it passes a screen with an opening of at the most 45pm.

6. Phosphorus containing steel powder as claimed in claim 1, wherein the phosphorus content of said ironphosphorus alloy powder is between about 14-16 percent by weight.

7. Phosphorus containing steel powder as claimed in claim 1, wherein the composition of said ironphosphorus alloy powder corresponds to that of Fe P, the relative proportions of said iron-phosphorus alloy powder and said powdery steel being such that the phosphorus content of the mixture is between about 0.3 and 1.5 percent by weight.

8. Phosphorus containing steel powder as claimed in claim 2, wherein said solid lubricant powder is zinc stearate powder.

9. Phosphorus containing steel powder as claimed in claim 2, characterized in that it also contains 0.005 to 0.02 percent by weight ofa thin mineral oil for the prevention of segregation. 

2. Phosphorus containing steel powder according to claim 1, characterized in that it also contains at the most 1.5 percent by weight of a solid lubricant powder.
 3. Phosphorus containing steel powder according to claim 1 characterized in that it also contains 0.005 to 0.02 percent by weight of a thin mineral oil for the prevention of segregation.
 4. Phosphorus containing steel powder according to claim 1, characterized in that the iron-phosphorus alloy particles are bonded to particles of said powdery steel by sintering and subsequent comminution for the prevention of segregation.
 5. Phosphorus containing steel powder as claimed in claim 1, wherein the particle size of said iron-phosphorus alloy powder is such that it passes a screen with an opening of at the most 45 Mu m.
 6. Phosphorus containing steel powder as claimed in claim 1, wherein the phosphorus conteNt of said iron-phosphorus alloy powder is between about 14-16 percent by weight.
 7. Phosphorus containing steel powder as claimed in claim 1, wherein the composition of said iron-phosphorus alloy powder corresponds to that of Fe3P, the relative proportions of said iron-phosphorus alloy powder and said powdery steel being such that the phosphorus content of the mixture is between about 0.3 and 1.5 percent by weight.
 8. Phosphorus containing steel powder as claimed in claim 2, wherein said solid lubricant powder is zinc stearate powder.
 9. Phosphorus containing steel powder as claimed in claim 2, characterized in that it also contains 0.005 to 0.02 percent by weight of a thin mineral oil for the prevention of segregation. 